This application is related to a ROADM system.
Submarine optical networks form the backbone of global communication networks. Submarine optical networks usually consist of main trunks that connect the trunk terminals, which are point-to-point cable links with in-line optical amplifiers to boost the signal power, and branch path that connect to other branch terminals. A branching unit (BU) is a network element that splits the signal between the main trunk and the branch path and vice versa. This allows the signals from different paths to share the same fiber, instead of installing dedicate fiber pairs for each link. The signal splitting and combining function of the BU is usually performed optically, therefore the BU has the similar function as the optical add/drop multiplexer (OADM) in the terrestrial WDM networks. FIG. 1 shows the schematic of existing submarine communication system that consists of two trunk terminals, one branch terminal and one BU that links the main trunk with the branch path. Bidirectional fiber transmission is illustrated. Note that there are there are two pairs of fiber between the BU and the branch terminal. One pair is used to connect Trunk Terminal 1 to the branching terminal, while the second pair is used between Trunk Terminal 2 and the branch terminal. This allows the reuse of the same wavelengths in the TT1-BT path and the TT2-BT path.
Typically, a BU consists of two subcomponents, one is called power switched branching unit (PSBU), and the other is an OADM unit. It's illustrated in FIG. 2. The PSBU is inserted in the main trunk, and has several 1×2 switches. It can decide whether there will be add/drop to a branch terminal from this point. If there is no branching path connected, or if add/drop to the branch terminal is not required, the switches will send the signals directly along the trunk path (FIG. 2(a)). This is useful to pre-set a branching point for future use.
If add/drop is required, and OADM unit is connected to the PSBU, and the 1×2 switches in the PSBU are switched to send the signal to/from the OADM unit, as shown in FIG. 2(b). Inside the OADM unit, a wavelength add/drop subsystem connects the 4 fiber pairs (or 3 pairs, if there's only one branch fiber pair), and performs wavelength add/drop function according to the network requirement. These 4 fiber pairs and the respective ports are named A, B, C, and D in this document, where ports A and B are connected to the main trunk through the PSBU, and ports C and D are connected to the branch terminal. There are optical signal monitors at each input port, and optical amplifiers at the input and output of each port. The 4-input-4-output OADM unit can be redrawn as FIG. 3.
The key task of designing a BU is to design the optical add/drop subsystem. Therefore in the remaining sections of this document, we ignore the components that are not related to switching or reconfiguration (such as amplifiers, power monitors, and power supplies), and focus on the optical add/drop subsystem (OADM subsystem) design.
Conventional BU and the submarine network have fixed, pre-determined wavelength arrangement, therefore no reconfiguration is required. However, the traffic in the global communication network is becoming more dynamic as Internet-based traffic becomes more dominating. Therefore the wavelength reconfigurability is required for the next generation submarine network, with reconfigurable BU as the key enabling element.
Various submarine network BU architectures have been proposed to add reconfigurability and to achieve reconfigurable optical add/drop multiplexing (ROADM) function between the main trunk and branch path in submarine network. The OADM unit becomes a ROADM unit, and the OADM subsystem becomes the ROADM subsystem. These architectures and techniques include using tunable filters, filter array with switch, wavelength-selective switches (WSS), 2×2 switches in bidirectional demultiplexer-switch-multiplexer (DSM) system, 1×2 switches in split-and-select DSM system, 1:2 interleaver with source tuning, 2:2 interleaver with source tuning, etc. Ref [1] describes these architectures, and compares their various features, including reconfigurability, number of branch fiber pairs, latching capability, and broadcasting feature, etc. It also contains more detail reference for each architecture.
Among these architectures, WSS-based architecture provides the highest level of reconfigurability (2K configurations can be achieved, where K is the number of WDM channels at the narrowest channel spacing acceptable in the system). (In comparison, the tunable filter-based architectures can deliver K configurations; the 2×2 switch-based architecture can deliver 2B configurations, where B is the number of wavebands, which is a few times smaller than K; and the interleaver-based architecture can deliver 2K/2 configurations if there's only one branching split, and fewer configurations at higher split numbers.)
WSS is also the key optical component for wavelength switching in the terrestrial ROADM nodes, it has reached technology maturity in the past decade and is widely available commercially by multiple key optical component vendors, therefore it is most likely that WSS-based architecture will also be the main solution for submarine reconfigurable branching unit design. This invention is targeting the WSS-based ROADM branching unit. There are 3 key requirements for submarine branching unit design:
1. Reliable: Due to the physical location and environment (deep sea, ocean bottom), the required time and effort to identify fault and to repair damages in submarine networks is much greater than in the terrestrial network. Therefore the reliability of the branching unit equipment is very important. Several measures are taken to address the reliability issue. For example, submarine-grade devices are used, these devices have low Failure in Time (FIT) rate (which is the number of failures that can be expected in one billion device-hours of operation). Also, it's desirable to use only passive device or device with latching feature (which means that the switches will maintain their switching setting even after the power is turned off or cut). Since most of the reconfigurable BU require active switches to provide reconfigurability (except very few examples such as in [2] where the reconfigurability is controlled at the terminal node), switches with latching feature is highly desirable. However, despite technology proposals such as [3], latching WSS is still not commercially available yet. Therefore another step to increase the BU equipment's reliability is to add redundancy, so that when part of the hardware fail, the BU can maintain all or partial functions.
2. Security: Secure data delivery is another important issue in submarine optical network design. Since submarine networks usually connect multiple countries, the possibility of a terminal (country) receiving non-designated data between other terminals (countries) is a serious security risk and should be prevented. This is even more critical since most submarine network is owned by a consortium of multiple companies and organizations, with ownership distributed among different countries. It is unlikely for a single country or a single company to manage the entire network and control the security setting, especially since the hardware are located at each participating country. Secure data delivery means that only the intended channels will reach each destination terminal (no matter it is a branch terminal or trunk terminal), and thus the data and information carried in each WDM channel cannot be received at unintended terminal.
3. Low cost: Due to the fact that high reliability submarine-grade devices are much more costly than regular device, and due to other submarine network-specific restrictions and requirements (such as long transmission distance, limited electrical power supply and limited space), the submarine branching unit hardware is more expensive than terrestrial ones. Therefore a design requirement is to keep the cost down as much as possible while meeting the other requirements. In reconfigurable branching unit, the most costly device is the WSS. Therefore keeping the number of WSS to the minimum is one of the design goals. For example, in the bidirectional, 2-branching-pair OADM unit shown in FIG. 3, a target is that the total number of WSS does not exceed 4 (one per path). This will also reduce the electrical power requirement and hardware footprint, which are both scarce in branching unit, it will also reduce control complexity, which leads to better reliability. These are all beneficial to branching unit equipment.
Since all these 3 requirements are important, they need to be met concurrently. However, it is challenging to satisfy all these requirements at the same time. For example, providing redundancy requires additional backup hardware, this is in contradiction to the requirement to keep the hardware cost low. In another example, having secure data delivery means that wavelength-selective device (such as WSS) should be used instead of passive splitting (broadcasting) device (such as optical splitter), but the cost of WSS is more costly then splitter. This invention is aimed at addressing this challenge, namely to meet all these requirements concurrently.
Since the ROADM subsystems in most submarine branching units are 4-input-4-output ROADM (such as in FIG. 3, with full reconfigurability), the target of the ROADM subsystem design can be described more specifically as: Besides meeting the fundamental requirement of providing full reconfigurability through using WSS, the 4-input-4-output ROADM BU should satisfy the following 3 requirements: (A) There are only up to 4 WSS devices in the ROADM unit; (B) When up to 2 WSS fail, the ROADM unit can maintain all the switching (add/drop/through) functions (i.e. with redundancy); (C) Secure data delivery can be maintained at all time, even during device failures. In this invention, this configuration and target is addressed, however the principle can be applied to other configurations (such as 3-input-3-output ROADM).
FIG. 4 shows the most common configuration of 4-input-4-output WSS-based ROADM subsystem. At each input, an optical splitter is used to split the signal to two paths, one for the opposite trunk terminal, the other for the branch terminal. At each output, a WSS is used to select only the appropriate signal for the destination terminal, therefore it achieves both full reconfigurability and secure data delivery functions. The 4-WSS limit is also satisfied. However, if any single WSS fails, the ROADM function cannot be fully maintained.
A similar configuration is called switch-coupler (SC) configuration, as shown on FIG. 5, where the WSS's are placed at the input end (1×2 WSS's, instead of 2×1 WSS's), and the optical couplers (same device as optical splitters) are placed at the output end. This is the same configuration as in FIG. 8 of Ref [1]. This configuration has the same feature as the one on FIG. 4: full reconfigurability, secure data delivery, no more than 4 WSS, but cannot maintain ROADM function when any WSS fails.
Therefore it can be noticed that the main limitation of the current ROADM design is that it cannot maintain ROADM function during WSS failure. In other words, providing redundancy protection is a key feature to be added. And it has to be done without requiring additional WSS hardware and without sacrificing secure data delivery feature.
One conventional design adds redundancy is to use 2×1 switches and additional fiber connections, as shown on FIG. 6. Firstly, there is an additional splitter at the input from Trunk Terminal A. The input signal is split into two paths, one is the original path similar to FIG. 4, the other is sent to Switch 2 for redundancy purpose. When WSS 1 fails, Switch 1 is set so that the B output takes signal from WSS 2 (which is reconfigured accordingly to provide traffic to Trunk Terminal B), and Switch 2 is also set to take the input A signal from the redundancy splitter for output to Branch Terminal D. If WSS 2 fails, WSS 1 and Switch 1 remain unchanged to provide traffic for Trunk Terminal B, but Switch 2 is switched to take the input A signal from the redundancy splitter for output to Branch Terminal D. Similar protection is used for the section in red for WSS 3 and WSS 4.
While being able to provide redundancy during single WSS failures or some cases of simultaneous two WSS failure (one from WSS1 and WSS2, and one from WSS3 and WSS4), this solution cannot provide traffic security to branch terminal during WSS failure, since the traffic to the relevant branch terminal comes from the input splitter from the trunk terminal and does not go through the selection/filtering process of WSS. Also it cannot handle the case when both WSS1 and WSS2 fail at the same time, or when both WSS3 and WSS4 fail at the same time.
Another conventional design uses additional 4×1 WSS as well as four optical splitters, two 2×2 switches, two 2×1 switches, and one 1×2 switches, as shown on FIG. 7. All input signals are split into two paths, one normal working path and one for backup. The 4×1 WSS works with other optical switches to provide WSS function in case one of the four original WSS fails. The advantage of this design is that it can maintain traffic security during single WSS failure. However, it requires additional WSS (and 4×1 WSS is usually more costly than 2×1 WSS), and it cannot handle more than one WSS failure at a time.